In a known method, a load applied to a wind turbine blade is measured on the basis of a strain of the wind turbine blade, to perform operation control for load reduction or to monitor a state of a wind turbine blade.
For instance, Patent Document 1 discloses obtaining a strain of a wind turbine blade using a sensor mounted to the wind turbine blade, and calculating a load on the wind turbine blade on the basis of a function representing a relationship between the strain and the load on a wind turbine blade.
Further, Patent Document 1 discloses calibrating the above function representing a relationship between strain and load on a wind turbine blade, on the basis of measurement data of a strain obtained by a sensor and a theoretical load in a no-wind condition calculated on the basis of a given blade weight and a pitch angle and an azimuth angle (a rotational position of the blade) of the wind turbine blade.